Well, there's fun and there's practical. Were I a NATO or SEATO nation and wanting to bolster my air force, then I think there's two ways you could approach it. There's the "fun" and there's the practical.
"Fun" options that would be at the top of the list for me would be:
F-15C Eagle
U-2S Dragon Lady
CH-53E Super Stallion
AV-8B Harrier II
EA-6B Prowler
B-1B Lancer
C-5M Super Galaxy
I call them fun because I like them, but they're too old and/or big to be practical for most countries not operating the type in question. Examples of the latter would be Israel buying surplus F-15C/Ds or Italy buying some more Harriers and pushing back the F-35B purchase.
You'd see the same for other older airframes. Most of the customers lining up for "legacy" Hornets would be existing users. Malaysia, Finland, maybe even Canada, Spain, or Switzerland.
Realistically, the most popular older aircraft that I think "new" users would take an interest in are the ones with a wide user base and a substantial support network. Think things like the F-16C, P-3C, C-130H, UH-60, AH-64D, CH-47D. Boring, I know, but you might see some more unusual combinations if there were good enough deals on the market. Think stuff like Kazakh C-130s, Vietnamese P-3 Orions, and South African UH-60s or Chinooks.
Outside of F-16s and Apaches, most US combat aviation assets would not be practical for most users for one reason or another. Age, size, cost, complexity, airframe hours, etc. Besides, as was pointed out on this thread:
Every Single Plane in the U.S. Air Force in One Chart (think of it like a shopping list!)
...the USAF is actually pretty light on combat assets, proportionally. And, in the stated scenario, the US isn't becoming pacifist, just isolationist, so most of the assets on offer would be those related to power projection.
In short, I think the sales catalog will be dominated by tankers, transports, and ISR assets. That should be fine, since those are the areas that US allies would find they are weakest in if they were to withdraw their support. So, you may see countries adding second-hand E-2 Hawkeye and E-3 AWACS assets, really cheap KC-135s getting snagged by fairly small air forces (like Chile's recent KC-135 buy), and anyone that was considering adding MPA picking up some well-used P-3 Orions. Far more in demand, however, would be the various airlift and transport aircraft that the US would be divesting themselves of. You'd see existing users bolstering their legacy C-130 fleets, other countries getting some used C-130Js with a lot of life and hours left in them, and—the big one—C-17s back on the market! I see a lot of countries that were kicking themselves for missing that opportunity when they were in production snatching up a handful, especially NATO and Gulf countries.
If the US permitted sale to civilian operators, you might see Antonov and Ilyushin hit hard as a ton of civilian C-17s and C-5s hitting the market, much like it was flooded with C-47s after WWII.
Cheers,
Logan